Modern (endogenous) growth theory tells us that knowledge is crucial for the sustained growth of high-income economies. Against this background, the paper provides a survey of theoretical and empirical findings highlighting the question of how geographically limited knowledge diffusion can help to explain clusters of regions with persistently different levels of growth. It discusses this topic in two steps. First, the theoretical concept of knowledge spillovers is outlined by discussing the different types of knowledge, the spatial dimension of knowledge spillovers, and the geographical mechanisms and structural conditions of knowledge diffusion. Second, it analyses the empirical evidence concerning the theoretical propositions.